The present invention relates to ophthalmic instruments, and in particular is concerned with those ophthalmic instruments whose primary purpose is the testing of various aspects of the field of vision of the human eye.
Visual field testing has usually required dual instrumentation regardless of the technique of perimetry. This is because central field testing requires a larger and more magnified examination of field defects than does peripheral field testing. Magnification is routinely and easily achieved by increasing the test distance, and so we usually wind up with a central field measuring device which is operated at a large distance (often 1000 mm), and a separate peripheral field measuring device which is operated at a short distance (often 330 mm).
Within the past 15 years, a number of attempts have been made to produce a single instrument for measuring both the central and the peripheral field of vision. One approach that has been taken, was to develop devices which tested the peripheral field at the same large distances used for the central field. However, this resulted in large, bulky and inconvenient instrumentation. These devices were so bulky in fact, that some of the peripheral field often had to be sacrificed, thereby defeating the purpose of the device itself.
A second approach to the problem of dual instrumentation has been based upon an erroneous argument. Peripheral field testing must be performed about the patient's eye, and therefore usually employs a device with some sort of curved surface for testing. However, central field testing need only be performed in front of the patient's eye, and for convenience has usually employed a device with a flat surface for testing. Because peripheral and central field testing, have become so associated with curved and flat testing surfaces respectively, we have seen devices which claim to test both peripheral and central fields by merely utilizing both curved and flat surfaces. Unfortunately, these surfaces have always been set at the same distance from the patient's eye and have either been too bulky for convenience when the testing distance was large, or inadequate for central field testing when the testing distance was small.